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putting an RPZ underground

I have been reading the past post regarding installing a Reduced Pressure backflow preventer inside and outside, but not underground. I would like to put RPZ outside and out-of-sight. Has anyone done this before?

I have seen that in commercial installations the RPZ is typically enclosed in a metal box that sits on top of a concrete pad. These metal boxes, however, very expensive ($500~$1000).

Is it possible to put an RPZ underground in a very large (bigger than a jumbo) valve box? The following conditions would be maintained:

condition-1) the RPZ will be at least 18" off the surface
condition-2) a sump to contain contain the backflow will be provided

If it is possible to install the RPZ underground, then have the following questions:

question-1) how big should the valve box be?
question-2) how large should be sump be?

Just read the installation specs from Febco. It states that some approval agencies do not permit the installation of an RPX in a pit. If the approval agency does permit it, then proper drainage must be provided to prevent cross-contamination.

This answers the question whether or not an RPZ can be installed underground.

The question still remains how large the sump should be. Could this be determined by calculating the total volume of water that would be in the mainline and all the laterals?

After searching the web I did find several references on the subject. In general, placing an RPZ in a pit is not recommended. It is possible to place an RPZ given proper drainage. In several cases drainage must be to daylight i.e., a simple sump is not good enough.

In my town, Hollis, NH there is no regulation regarding placing an RPZ in-ground. In fact, there is no requirement that a backflow preventer is necessary at all. However, what may be required somewhere else probably makes good common sense everywhere elese.

To summarize, I plan on using an RPZ that is installed above-ground. Instead of an expensive enclosure, perhaps I will build a "dog house" like enclosure to match the house.